Biaxially oriented polypropylene (BOPP) films are typically used for packaging, decorative, and label applications and often perform multiple functions. In a lamination, they provide printability, transparent or matte appearance, or slip properties. The films sometimes provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. The films sometimes provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or by laminating.
However, in recent years, interest in “greener” packaging has been strongly developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gas generation. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable or compostable as well. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable resource—is one of the more popular and commercially available materials available for packaging film applications. Such bio-based polymers used for packaging applications can also be degradable and/or compostable in certain situations.
For such a bio-based polymer to be fit-for-use for many snack food packaging applications, it is desirable that the bio-based polymer film match as many of the attributes possible that BOPP is well-known for, such as heat sealability, printability, controlled COF, metallizability, barrier, etc. For example, for high barrier packaging, metallized oriented PLA films should demonstrate good oxygen and moisture barrier properties. For metallized oriented PLA in particular, good oxygen barrier property is generally easily achieved due to the polar nature of PLA, which provides good hydrogen-bonding of the polymer molecules. However, this polar nature tends to be detrimental for achieving high moisture barrier. Without being bound by any theory, the thought is that water molecules—being polar themselves—may more easily migrate through a polar polymer film than a non-polar polymer film.
In recent applications of biaxially oriented polylactic acid (BOPLA) films used in place of BOPP in snack-food packaging applications, it has been noted that the higher stiffness and modulus of BOPLA films versus BOPP films, have resulted in a “noisier” package, particularly for flexible bag packaging for salty snacks such as corn-based chips or potato chips. Many consumers have noted that chip packaging utilizing a lamination of a BOPLA print film and a BOPLA metallized gas barrier film is significantly louder during handling (i.e. opening of the chip bag, reclosing of the chip bag) than traditional packaging using a lamination of a BOPP print film and a BOPP metallized gas barrier film; or even an hybrid package lamination including a BOPP print film and a BOPLA metallized gas barrier film. For some consumers, the increased loudness or noisiness of the BOPLA/BOPLA laminate package is objectionable compared to traditional BOPP/BOPP laminate packages. Of note, however, is that if a laminate package is made using a BOPP film laminated to a BOPLA film, the noise generated could be acceptable to the consumer. However, such “50%” bio-based laminate does not satisfy the above requirements of being “green” because the proportion of sustainable, renewable materials is not as high as desired; furthermore, it does not bio-degrade or compost due to the high percentage of OPP in the laminate structure. Thus, it appears that to reduce noise in an all-BOPLA laminate package structure—and also retain its high renewable material content and compostability—it is necessary not only to “soften” but also to “quiet” the BOPLA film. Quieter BOPLA films for packaging applications that maintain compostability of the package and suitable processability are described.
Modifier and additives have been added to BOPLA films to reduce the modulus and stiffness of the BOPLA films. U.S. Pat. No. 7,128,969 describes a film composed of a base layer of PLA with a minority component of a thermoplastic or polyolefin such as polypropylene or polyethylene, typically less than 1% by weight of the base layer. Such a formulation is particularly suitable for thermoforming or biaxial stretching by means of pneumatic drawing or other mechanical forming. However, this patent does not include formulations that also reduce the noise of the film, and maintain compostability and suitability for processing.
U.S. Pat. No. 5,908,918 describes an impact-modified PLA packaging film using a degradable impact modifier and plasticizer. The impact-modifier is preferably combined with the plasticizer to achieve the desired improvement in impact strength and flexibility. The use of low molecular weight, migratory plasticizers, however, can be detrimental to other useful properties desired in packaging films such as metallizing for high barrier or high-definition printing of the packaging film for graphics and advertising. This patent does not contemplate achieving highly flexible PLA-based packaging films using components that do not contain a plasticizer and which may not affect metallizing or printing characteristics.
Similarly, EP Patent Application 08739216 describes a film formulation to produce softer PLA articles using a vegetable oil-derived plasticizer with the PLA resin. However, such vegetable oil-based materials may be migratory and cause problems with metallizing or printing of such a modified substrate.
The article “Improving PLA Mechanical Properties by the Addition of Oil,” Polymer Engineering & Science (2010), vol. 50, issue 3, pp. 513-519, describes using oils to toughen PLA films. Upon addition of specific oils, PLA film modulus decreases and ductility improved. However, such oils are migratory and can cause processing problems as well as poor properties for metallizing and printing.
The article “Technology Focus Report: Toughened PLA,” Natureworks® LLC (2007) technical bulletin, describes the use of various types of elastomers—including non-degradable polyolefin elastomers—to improve toughness and impact properties of PLA articles. However, such elastomers are preferably added at amounts of 10-30 wt % to achieve improvements in toughness and such amounts can worsen compostability.
U.S. patent application Ser. No. 12/161,967 describes a film formulation including PLA and polyether-polylactic acid copolymer. The addition of an amount of polyether-polylactic acid copolymer softens the PLA is rather large, about 20-50 wt % of the film.
U.S. Pat. No. 7,214,414, WO 2007046174 and JP 200531998 disclose films including low Tg polymers that allow the films to have some suitable properties for packaging films. However, the films are mainly described as being suitable for blown film processing. These references do not provide sufficient disclosure of films suitable for sequential processing or a simultaneous biaxially oriented PLA film-making processing. Furthermore, these references do not describe how to achieve the quietness and sufficient process suitability.
JP4210492 discloses a biaxially oriented PLA film having 15-50 wt % of aliphatic polyester to achieve good die cutting properties etc. U.S. patent application Ser. No. 09/932,523, publication 2003/0039775 A1 describes polylactic acid-based films including a toughening additive of 5-40 wt % of the core. The toughening additives are shown to improve elongation-to-break property of the films but also show a high degree of heat shrinkage and less thermal stability which can be detrimental for many packaging applications.
U.S. Pat. No. 6,649,103 describes mono-axially stretched polypropylene-based films that contain phases of a second polymer such as EVOH and compatibilizing materials. Such structures form a lamellar multi-phase structure and show improved gas barrier properties. However, PLA-based materials are not considered as the matrix polymer. Second phase materials such as PBAT and PHBV are also not considered.